Similar pressure sensors to that mentioned at the start are known for example from WO 2006/089446. In this pressure sensor, a diaphragm is situated in the front region of the glowplug, which diaphragm seals off the interior of the housing from the combustion space. A prestress can also be applied to the sensor by means of this diaphragm.
The reference symbols are identical for all the figures.
FIG. 1 shows a schematic illustration of a pressure sensor 1 in section according to the prior art from WO 2006/089446.
This pressure sensor 1 comprises a housing 2, in the front region of which a pressure element 3, in this case configured as a glowplug, is inserted, which projects out of the housing 2. Downstream of the pressure element 3 there is a force transmission element 21, downstream of which a sensor element 5 is arranged. Downstream of this sensor element 5 there is a fixing element 22, which bears against a connection element 23 at the rear. The connection element 23 is attached at its rear end to a shoulder 24 of the housing 2. The force transmission element 21, sensor element 5 and fixing element 22 together form a measuring element 4. The sensor element 5 is in the prestressed state in the pressure sensor 1.
To attach the pressure sensor 1, configured as a pressure-measuring glowplug, in a bore provided for it in a cylinder head, the housing 2 generally has a thread 25 and a hexagon 26 as a working surface for a tool. In the rearmost region of the glowplug there is the exit for the wiring 27, which also includes lines for the measuring element 4.
in the front region of the pressure sensor 1 there is a diaphragm 7 which seals off the interior 6 of the housing for example from the combustion space. A prestress can also be applied to the sensor element 5 by means of this diaphragm 7. The diaphragm 7 is in this embodiment attached at its first end, the inner end, permanently to the pressure element 3 and at its second end, the outer end, to the housing 2.
A pressure in the combustion space or in the chamber 29 is transmitted to the sensor element 5 by means of the pressure element 3 and the force transmission element 21. The measuring element 4 is carried by a pin 28 which runs centrally through it. This pin 28 has no effect on the force transmission. A glow current wire 30 which, in the case of a measuring glowplug, can heat the pressure element 3, also runs in this pin.
The force path 20 is shown with a dotted line on one side in this figure. It runs from the pressure element 3 via the force transmission element 21, via the sensor element 5, via the fixing element 22, via the connection element 23, via the rear shoulder 24, and finally via the housing 2 via the diaphragm 7 back to the pressure element 3. If one of these bearing faces is not shaped precisely and has a resilient bearing which can absorb energy when loaded, a measurement error is produced.
The force transmission from the pressure element 3 to the force transmission element 21 creates the most problems in this embodiment. The material of the pressure element 3 is generally different from the material of the diaphragm. It is very difficult and expensive to produce a pressure element 3 with the required flatness.
Alternative embodiments are also known in which what is known as an “anti-strain” sleeve is applied around the measuring element. This sleeve supports the rear region of the measuring element and is connected to the housing at the front. This means that the sensor is not sensitive to stresses and forces which act on the housing.
It has been shown that a sensor according to the mentioned document WO 2006/089446, in which an anti-strain sleeve was applied, still does not operate satisfactorily. If the force transmission from the pressure element to the force transmission element is elastic, an error creeps in during a measurement as a result.